Impact of x rays on the sensitivity of CR-39 detectors to 2.4-MeV protons

Solid-state nuclear track detectors, such as CR-39, lose sensitivity when subjected to doses of x rays on the order of 1 Gy in nuclear-fusion relevant experiments. As a result, the formed tracks have, in general, smaller maximum radii and shallower maximum depths. Presented here are experiments using atomic force microscopy to measure the CR-39 sensitivity parameter Vt/Vb, the ratio of track to bulk etch rates, for ∼2.4 MeV protons. The measurements revealed that absorbed x-ray doses of the order of 1 Gy reduced the CR-39 track diameters by a factor of 3 from ∼1200 to ∼400 nm and reduced the track depths by an order of magnitude from ∼550 to ∼50 nm. The corresponding change in the sensitivity parameter was inferred and found to have substantially decreased from ∼1.1 to ≲1.01. Doses in excess of 0.5 Gy reduced the track diameter, at an etch time of 2 h, below the 1.0 μm limit typical of optical microscopy methods used when scanning large, i.e., ≳25 cm2, samples of CR-39. The findings suggest that the mechanism of sensitivity loss is driven by x-ray induced cross-linking of the bulk CR-39. The enhanced cross-linking reduces the solubility of the material along charged-particle trajectories compared with that of the bulk material. Representative absorbed doses of between 0.01 and 1 Gy in CR-39 on OMEGA and NIF in typical experimental configurations are then compared with the results.